The present disclosure relates generally to identifying and/or analyzing variations in a measured physiologic parameter, such as blood oxygen saturation (SpO2) measured using pulse oximetry. More particularly, the present disclosure includes embodiments directed to analyzing variations in SpO2 values and/or variations in SpO2 trend data that are smaller in magnitude than the accuracy, display precision, and/or calibration of the measurement.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Pulse oximetry is used to continuously monitor the oxygen content of patients' blood in various settings (e.g., operating rooms, delivery rooms, and so forth). Specifically, pulse oximetry uses light waves to indirectly measure the arterial blood oxygen saturation of patients. For example, in operation, conventional two wavelength pulse oximeters emit light from two emitters (e.g., light emitting diodes (LEDs)) into a pulsatile tissue bed and collect the transmitted light with a detector (e.g., a photodiode). The detected light may then be utilized to estimate a level of oxygen saturation in the blood that is present in the tissue bed. The emitters and detector may be positioned in various orientations for different types of pulse oximetry. For example, in transmission pulse oximetry, the emitters and detector are positioned substantially opposite one another (e.g., on opposite sides of a patient's finger), while in reflectance pulse oximetry, the emitters and detector are placed adjacent to one another. The emitters and detector are typically housed in a sensor which connects to pulse oximeter electronics.
The “pulse” in pulse oximetry comes from the time varying amount of arterial blood in the tissue bed during the cardiac cycle. The processed signals from the photodetector create the familiar plethysmographic waveform due to the cycling light attenuation caused by the varying amount of arterial blood that the light from the emitters passes through. With regard to conventional two-wavelength pulse oximeters, at least one of two LEDs emits light at a wavelength at some point in the electromagnetic spectrum where the absorption of oxyhemoglobin (O2Hb) differs from the absorption of reduced hemoglobin (HHb), and the other of the two LEDs emits light at a wavelength that is at a different point in the spectrum where the absorption differences between HHb and O2Hb are different from those at the first wavelength. The use of these differing wavelengths facilitates estimation of blood oxygen saturation.
Typically, pulse oximeters utilize one wavelength in the red part of the visible spectrum near 660 nanometers (nm), and one in the near infrared part of the spectrum in the range of 880 nm-940 nm. Photocurrents generated within the detector are detected and processed for measuring the modulation ratio of the red to infrared signals. This modulation ratio has been observed to correlate well to arterial oxygen saturation. Pulse oximeters and pulse oximetry sensors are empirically calibrated by measuring the modulation ratio over a range of in vivo measured arterial oxygen saturations (SaO2) on a set of patients, healthy volunteers, or animals. The observed correlation is used in an inverse manner to determine SpO2 based on the real-time measured value of modulation ratios. It should be noted that, as used herein, SaO2 refers to the in vivo measured functional saturation, while SpO2 refers to the estimated functional saturation using pulse oximetry.
Traditional uses of pulse oximetry are based on rounded values of SpO2. Indeed, because SpO2 values are generally only accurate to about 1 or 2%, the SpO2 values are typically used after they have been rounded to the nearest 1%. Further, traditional uses of pulse oximetry may not typically benefit from more precise calculation. For example, pulse oximetry has traditionally been used on patient populations where arterial blood oxygen saturation is generally greater than 90%. In other words, pulse oximetry has traditionally been used in patient populations, wherein the functional hemoglobin in the arterial blood includes at least 90% oxyhemoglobin and 10% or less reduced hemoglobin. In such patient populations, oxygen saturation seldom falls below 80%, and such low values are generally indicative of an unhealthy condition that warrants intervention. Thus, in this and similar situations where pulse oximetry has typically been employed, a high degree of precision in the estimate of blood oxygen saturation based on traditional pulse oximetry has not generally been considered to be clinically relevant.